1. Field of the Invention
In general, the present invention relates to a method of reshaping a patterned photoresist. The surface of the patterned photoresist material is “trimmed” and “refinished”, to improve the pattern profile and surface finish of the patterned photoresist. The method is particularly useful in the preparation of reticles which are used in combination with an exposure tool to image photoresist layers overlying a semiconductor substrate. The method may also be used to prepare a photoresist masking layer which is used to transfer a pattern to an underlying hard masking material on a semiconductor substrate. The method is useful for reshaping patterned i-line photoresists and is particularly beneficial when the patterned photoresist is a deep ultra violet (DUV) photoresist.
2. Brief Description of the Background Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. The miniaturized electronic device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor substrate rather than by direct write on the semiconductor substrate, because of the time economy which can be achieved by blanket processing through a patterned masking layer. With regard to semiconductor device processing, the patterned masking layer may be a patterned photoresist layer or may be a patterned “hard” masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure to be patterned. The patterned masking layer is typically created using another mask which is frequently referred to as a photomask or reticle. A reticle is typically a thin layer of a chrome-containing material deposited on a glass or quartz plate. The reticle is patterned to contain a “hard copy” of the individual device structure pattern to be recreated on the masking layer overlying a semiconductor structure.
A reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today's semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation material becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include layers of chromium, chromium oxide, and chromium oxynitride. The photoresist layer upon which the direct writing is carried out is frequently a chemically amplified DUV photoresist material today, because of pattern dimensional requirements.
A patterned chemically amplified DUV photoresist layer frequently exhibits a “foot” at the bottom of the pattern profile, where the photoresist layer interfaces with an underlying ARC layer on the chrome-containing surface. Some developed photoresists exhibit a “t”-top profile. In addition, the surface of the patterned photoresist layer typically exhibits standing waves, due to reflections which occur during the direct writing on the photoresist layer, despite the presence of the underlying ARC layer.
To provide a photomask or reticle capable of accurately producing critical dimensions of 0.15 μm or smaller, it is highly desirable to trim and resurface (reshape) the patterned DUV photoresist to remove a foot, a t-top, or standing waves remaining after patterning.
FIG. 1A shows a schematic of a cross-sectional view of a typical starting structure 100 used to form a reticle, including, from bottom to top, a quartz substrate 102, overlaid with chrome-containing layer 104, overlaid with an ARC layer 106, and a photoresist layer 108. As shown in FIGS. 1B and 1C, after patterning of the photoresist layer 108, there is often a “foot” 110 extending from the lower portion of patterned photoresist layer toward the surface 116 of ARC layer 106. The presence of a foot (feet) 110 makes it difficult to maintain control of the critical dimensions during subsequent etch transferring of the photoresist pattern through the ARC layer 106 and chrome containing layer 104. The foot also impacts the metrology capabilities of the lithographer.
FIG. 1C, which is an enlargement (from FIG. 1B) of a portion of the patterned photoresist layer 108 (with underlying ARC layer 106), shows a line 107 which exhibits “t”-topping 113 in the upper portion of line 107, feet 110 at the base of line 107, and ripples (standing waves) 114 on the sidewall 111 surfaces 112 of line 107. The “t”-topping 113 is believed to be caused by contamination/reaction which occurs at the upper surface of the photoresist layer during processing prior to development of the pattern. The standing waves 114 are generated by reflected radiation within the photoresist material, which occurs during the direct writing of the pattern into photoresist layer 108. The ARC layer 106 helps reduce the standing wave effect by reducing reflection back from underlying layers and device features into the photoresist layer 108, but standing waves are generated in varying degrees depending on the imaging system and the material composition of the particular photoresist. When the photoresist is a chemically amplified photoresist, transparency of the photoresist material is particularly high throughout the entire direct writing process; this results in increased reflectivity (greater than that for earlier i-line novolak photoresists), which increases the formation of standing waves 114.
Since most photoresists are comprised of organic materials, a plasma formed from oxygen (O2) gas has been used to remove residual photoresist material remaining on the exposed surface 118 of ARC layer 106 after photoresist patterning. This cleaning of the exposed surface 118 of ARC layer 106 is frequently referred to as “descumming”, since a scum of residual organic material remains over surface 118 after patterning of photoresist layer 108. A small reduction in the size of feet 110 may occur during the descumming process. However, since a plasma formed from O2 gas tends to be isotropic in nature, the feet 110 are not removed completely. Or, if the feet 110 are totally removed, this frequently causes a change in the critical dimension (an enlargement in the opened area of the photoresist) of the pattern in patterned photoresist layer 108 by the time the feet 110 are removed.
It would be desirable to provide an effective process for reshaping the patterned photoresist, which process removes the feet at the base of patterned sidewalls and removes the ripples (standing waves) from sidewall surfaces while enabling the control of the photoresist critical pattern dimensions.